Method for determination of pith location relative to lumber surfaces

ABSTRACT

The invention is a method of assisting lumber grading using an electro-optical scanning system. A first critical determination is determination of pith position of the log from which the lumber was cut, relative to the scanned faces. This then indicates knot orientation. With knot orientation known, knot size and position data also determined by the scanning system can be used to accurately estimate cross sectional area of the knots. From this point a tentative lumber grade can readily be assigned. Pith position is indicated by a number of factors including wane, knot counts on each of the faces and the presence or absences of spiky faces. A preferred form the scanner is one which measures localized wood fiber angles relative to three orthogonal axes. These fiber angle measurements reveal grain slope disturbances on the lumber which are indicative of many types of defects including knots. The system has been used for grading lumber at speeds as high as 350 m/min. with considerable accuracy.

This application is a continuation-in-part of application Ser. No.067,334, filed June 26, 1987, now U.S. Pat. No. 4,831,545.

BACKGROUND OF THE INVENTION

The present invention is an automated method for assisting lumbergrading. It involves determination of the pith location of the log fromwhich the lumber was cut relative to the lumber surfaces. Having thisknowledge, the orientation of knots within the lumber can be ascertainedand the cross sectional area of the knots more readily estimated. In itsmost preferred form, the method assigns a tentative grade to the lumber.

Several criteria are used in grading lumber products. Lumber to be usedfor furniture, mill work or similar products is graded on the basis ofappearance with relatively little consideration being given to defectswhich could affect strength. On the other hand, appearance is givenessentially no weight in grading lumber products intended for structuraluse. The lumber grades are assigned on the basis of the expectedstrength in bending, compared with that of similar defect-free products.Most grading rules in the United States are based on ASTM Test Method D245-81. This publication notes that lumber grades "designatenear-minimum strength and near-average stiffness properties on which tobase structural design."

Virtually all structural lumber today is visually graded. This is doneby examining all four faces and the ends of the piece and by noting thelocation as well as the size and nature of knots and other defects overthe entire length. As noted earlier, lumber grades are based on thestrength ratio of a structural member compared to the strength it wouldbe expected to have if no weakening characteristics were present. Thevarious factors that affect strength are knots, grain deviations such asdiving grain, shake, and checks or splits. Certain other factors such asvarious types of warpage, density, and the presence or absence ofsurface defects such as planar skip or wane also enter into the gradingprocess.

Most structural lumber in the United States is graded with theassumption that it will be used as a joist; i.e., with bending loadsapplied to one of the narrow faces of the lumber. Other grading rulesapply to lumber designated for use as planks; i.e., with the bendingload applied to one of the wide faces. Some lumber is graded for use incompression, wall studs being an example.

While all of the defects noted previously are important, the presence ofknots is generally the principal reason for downgrading lumber. To againquote from ASTM D-245 "Strength ratios associated with knots and bendingmembers have been derived as the ratio of moment-carrying capacity of amember with cross section reduced by the largest knot to themoment-carrying capacity of the member without defect. This gives theanticipated reduction in bending strength due to the knot. Forsimplicity, all knots on the wide face are treated as being either knotsalong the edge of the piece (edge knots) or knots along the centerlineof the piece (centerline knots)." Thus, in grading a piece of lumber thegrader must visually and very rapidly note the number, size and positionof knots, estimate their cross sectional area, and mentally determinehow much they will reduce the strength of the piece. In addition, hemust also factor in all of the other flaws which affect grade. He mustdo all of this in a period of only a few seconds, mark the grade on thelumber and then move on to the next piece.

Grading is a job that requires very high skill. Graders are routinelyexamined by a head grader from one of the industry supported gradingbureaus and are typically required to assign correct grade at least 95%of the time.

In the past, many attempts have been made at machine grading lumber.Probably the simplest of all methods is to bend the lumber as a plank inflexure and determine modulus of elasticity. Pieces that fall below agiven threshold level are diverted to the lower grades. Other moresophisticated methods involve scanners which determine the presence andlocation of knots and/or other defects and assign a grade on this basis.One group of scanners detects knots on the basis of their color incomparison to the adjacent wood. Stated otherwise, scanning is based ona gray scale with the darker colors representing knots and the lightercolors representing knot-free wood. One early method is disclosed inU.S. Pat. No. 3,120,861 to Finlay et al. Another group of automatedgraders also involves human intervention with the scanning system. Onesuch method is described in U.S. Pat. No. 3,329,181 to Buss et al. Herea human inspector marks the flaws prior to scanning and may inputinformation to the computer describing the nature of the flaw; e.g.,that it might be acceptable in some lumber grades and unacceptable inothers. Barr et al in U.S. Pat. Nos. 3,931,501 and 3,942,021 include intheir background a good description of prior art scanners which areassociated with a computer for decision making. The Barr scanners alsorequire human input for designating the defect type which must be markedon the board prior to scanning. In this system the scanner unit movesalong the workpiece and uses an ultraviolet light and photocell array todetect defects, including those marked by the inspector. U.S. Pat. No.4,163,321 to Cunningham relates closely to the system of Barr et al.

Another scanning system for grading lumber which requires humanintervention is described in U.S. Pat. Nos. 4,149,089 and 4,221,974 toMueller et al. The Mueller system uses a swept laser beam to examineboth faces of a board. A human inspector serially inputs the class offlaw to the computer as it passes the scanning station. The humaninspector may enhance a flaw for detection by the scanner or maysuppress certain characteristics which the scanner might detect as anobjectionable flaw. The computer then makes optimum cutting decisions bycomparing the board characteristics with an order file. U.S. Pat. Nos.4,149,089 and 4,207,472 to Idelsohn and U.S. Pat. No. 4,286,880 to Youngshow systems which are similar in many respects to those described byMueller et al.

The prior art systems noted to date are predominantly interested indetecting defects which affect the visual appearance of wood and littleemphasis is placed on defects which affect structural properties. Inthese systems the wood is primarly intended for use in furnituremanufacture and may be very high value hardwoods so that optimum cuttingdecisions for remanufacture can be of great economic importance.

A somewhat different type of scanner for wood is disclosed in PCTApplication No. WO85/00657. Here the scanners examine at least one endand one longitudinal surface. One set of sensors have coarse resolutionwhile another set has fine resolution. The fine resolution sensors arecapable of movement so that they can examine specific areas of thelumber noted by the other sensors. This also is a gray scale scanner.However, the inventors note that other defects besides knots, such aspith streaks, can be recognized.

A different scanning system is disclosed in U.S. Pat. No. 3,976,384 toMatthews et al. Here light is injected into the surface of lumber at onepoint and the emerging light is detected at a spaced apart location. Thesystem can detect defects such as knots, blue stain, and certain typesof rot. The inventors have noted that in clear (defect free) wood, lighttraveling across the grain is attenuated by a factor almost 50 timesgreater than light traveling along the grain. The method senses surfacefiber direction changes on the plane of the face being measured.However, it has been found to be unsuitable for use on the woods ofdeciduous, or so-called "hardwood," species.

Another type of system using microwave scanning is found in U.S. Pat.Nos. 4,123,702 to Kinanen et al and 4,607,212 to Jakkula. Kinanen useshigh frequency microwave energy to detect knots based on their differingdielectric constant from sound wood. Jakkula similarly uses microwaveradiation to detect knots by noting a mode or polarization shift fromTEM to TM which is detected by a receiver. This inventor notes thatknots can be detected in a wane edge but does not comment further on thethis observation.

Another group of Finnish inventors, Hirvonen et al, in U.S. Pat. No.4,482,250 scan a wood surface with a polarized beam of light having theplane of polarization either parallel to or perpendicular to thelongitudinal axis of the piece of wood. The light reflected from a pieceof sound wood is only partially depolarized while the light reflectedfrom a knot is almost totally depolarized.

Davis et al, in U.S. Defensive Publication No. T932,008 disclose anoptical system for measuring surface fiber direction in a moving webstructure. These authors note that when collimated light is projected ona fibrous web, the light will be reflected with greatest intensityperpendicular to the fiber axes. Reflectance values measured along andacross the machine direction of the web can be used to determine theaverage orientation of the fiber on the surface in two dimensions.

An advanced scanner which can determine fiber angle in three dimensionsfrom a surface examination is described in Matthews et al, U.S. Pat. No.4,606,645. This is commonly assigned with the present invention and isincorporated herein by reference. This Matthews invention measures fiberangle in a fibrous solid material relative to three mutually orthogonalreference axes. It is particularly well suited from measuring divinggrain and grain surface angle in wood. This scanner employs a laser beamwhich may be mechani-optically swept transversely across the surface ofa longitudinally moving piece of lumber. Reflections from the surfaceare measured by an array of photo detectors lying preferably in a commonplane around the illumination source. The method can very readily pickup aberrations in grain angle such as those caused by knots below thesurface or by visible knots. Additionally, it can readily detect otherdefects such as planar skip, wane, and splits.

Other scanners have been concerned with detection of wane on lumber.Wane is here defined as an original unsawn surface of the log whichtruncates one or more edges of the piece of lumber. U.S. Pat. No.3,983,403 to Dahlstrom et al describes a system for detecting andmeasuring wane. Another system which can recognize wane is described inU.S. Pat. No. 4,188,544 to Chasson.

While all of the above-noted inventions are useful in some way forscanning wood surfaces and providing quality information, none of themhave been developed into apparatus which can do structural grading atthe very high speeds required in a modern lumber mill. In fact, thosewhich require human intervention are in many cases no faster or moreaccurate than visual grading. The present invention has overcome theproblems inherent in the prior art methods and is capable of accuratelygrading structural lumber at the high speed found in modern sawmills. Inadvanced versions of the invention, no further human intervention isnecessary, while in simpler versions a tentative grade is proposed whichmay be approved or modified by a downstream human inspector. In eithercircumstance the amount of human labor and decision making involved isgreatly reduced over that required for manual grading.

SUMMARY OF THE INVENTION

The present invention in its broadest form is a method of assistinglumber grading. It comprises determining pith position of the log fromwhich the lumber was cut, relative to preselected faces of the lumber,whereby knot orientation may be determined. The method further includescomputing the orientation of the knots, determining the effectivediameter and location of any knots detected, and estimating their crosssectional area whereby a tentative grade may be assigned to the lumber.

The method involves conveying the lumber past a scanning station. Firstand second faces of the lumber are scanned to determine the presence orabsence of wane, and its location if present. The second face is definedas being opposite and essentially parallel to the first face. At leastthe first and second faces are further scanned to determine the numberand size of knots present on each face. Optionally, the other faces oredges may also be scanned. With the wane and knot information thussupplied by the scanners, the pith position of the log from which thelumber was cut may be computed, relative the scanned faces. With pithposition known, knot orientation may be readily determined. The methodfurther includes determining the effective diameter and location of anyknots which are detected. The orientation of the knots may be computedand an estimated cross sectional area determined. With this knowledge atentative grade can be assigned to the lumber.

In its preferred embodiment the method includes a scanning step whichcan measure localized grain direction as an indicator of the presence orabsence of any knots. Preferably this scanning method will determinelocalized wood fiber angle relative to three mutually orthogonal axes.These fiber angle measurements reveal grain slope disturbances on thelumber which are indicative of a number of defects which include but arenot limited to knots. Preferably this scanning step will also determinethe presence or absence of any spiky faces on the lumber being scanned.A spiky face is here defined as one which lies on or closely adjacent toa plane which is parallel to a plane defining a diameter of the originallog and is likely to contain elongated "spike" knots. Spiky faces mayshow an exposure of pith or indicate that the pith is included withinthe piece of lumber being examined.

Normally the scanning steps include making a multiplicity oflongitudinally spaced apart scans transversely across each face beingscanned. Scanning steps for wane and knot presence/absence may becarried out either sequentially or simultaneously. Preferably, in thepresent method, they are carried out simultaneously.

It is thus an object of the present invention to provide a method fordetermining pith position of the log from which a piece of lumber wascut, relative to one face of the lumber.

It is another object to determine pith position whereby the orientationof knots in the lumber may be readily determined.

It is further object to provide a method in which grain slopedisturbances on the lumber are indicative of defects including knots.

It is also an object of the present invention to provide a method forestimating location and cross sectional area of knots within a piece oflumber so that a tentative structural grade may be assigned to thepiece.

These and many other objects will become readily apparent to thoseskilled in the art upon reading the following detailed description ofthe invention taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a log from which six pieces of finishedlumber may be obtained.

FIG. 2 is a similar cross sectional view of a smaller log from whichonly three pieces of finished lumber are obtained.

FIG. 3 is a fragmentary view of a portion of the wide face of a piece offinished lumber showing an edge knot.

FIGS. 4A through D are cross sectional views taken along line 4--4 ofFIG. 3 showing four possible configurations of the knot within thelumber.

FIG. 5 is a fragmentary portion of a piece of finished lumber showing aspiky face.

FIGS. 6 and 7 are cross sections respectively taken along lines 6--6 and7--7 of FIG. 5.

FIG. 8 is an edge view of the lumber shown in FIG. 5 taken along thelines 8-8.

FIG. 9 is perspective view of a piece of lumber showing wane at onecorner.

FIG. 10 is a grid showing one way in which pith position can beassigned.

FIGS. 11 and 11A show a block diagram giving decision logic for thepresent method of assigned pith position.

FIG. 12 is a cross sectional elevation view representing one scanningstation.

FIG. 13 is a plot of signal strength versus time for the wane scannersused with the apparatus of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Most of the old growth timber in the United States has now been cut andsawmills are receiving logs having increasingly smaller averagediameters. The method of the present invention is most effectivelyapplied to small logs being processed in high speed automated sawmillsalthough it should not be considered as limited to these conditions.Many mills are no longer equipped to handle logs larger than about 50 cm(20 in.) in diameter. More typically mills, especially in the south andsoutheast portion of the country receive log mixes in which the largestlogs are rarely over about 35 cm (14 in.) in diameter. These logs aretypically processed by passing them through a first station in whichflat faces are created on both sides of the log to produce a cant. Thematerial removed is converted into pulp chips. When logs are largeenough, a pair of parallel band saws may remove one or more side boards.FIG. 1 indicates a log in which gang cant and two side boards areproduced, in addition to the chips. FIG. 2 shows a smaller log in whichonly a gang cant is produced. The cants taken from the center of the logare then normally processed in a single or double arbor circular gangsaw to produce a number of pieces of lumber. The slabs taken from theoutside faces are separately sent to a chipper where the useful fibercan be recovered.

Reference to FIG. 3 shows a piece of lumber containing an edge knot.When viewed only from the face shown, there is no information as to howa serious a defect the knot presents. Previous scanner grading methodswould assume that the knot is a cylinder projected through the piece oflumber and grade it accordingly. The present method makes no suchassumptions. FIGS. 4A through D show four possibilities for the edgeknot of FIG. 3. In FIG. 4A the knot originates within the piece. Thiscondition, in which the pith or growth center is enclosed within thepiece of lumber, is called "boxed heart." In FIG. 4B the knot merelynicks the edge of the lumber and does not create a serious reduction instrength. However, in FIG. 4C the knot passes entirely through thelumber and occupies a large portion of the cross sectional area. FIG. 4Dis essentially the condition that would have been postulated by earlierscanners, where the knot is an approximate cylinder passing through thelumber normal to both wide faces.

The four possibilities shown in FIGS. 4A through D each representdifferent knot positions and cross sectional areas and each affect thestrength of the board differently at the cross section along line 4--4of FIG. 3. The condition shown in FIG. 4C is obviously much more seriousthan that represented in FIGS. 4A or 4B.

Whereas those previous scanning systems in the prior art could not ineffect "see into the piece," the present inventors have discovered thatthis can be done quite readily by determining pith location relative toone of the faces of the log. FIG. 4A represents a boxed heart piece inwhich the pith is contained within the lumber. In FIGS. 4B and 4C, thepith is closest to the upper faces and in FIG. 4D closest to the lowerface, although this cannot yet be determined with the information onhand. More information is gained by looking for spiky faces such as arerepresented in FIG. 5. Here an actual pith streak is seen along theupper right portion of the drawing and it is evident that the pith heremust be closest to the upper face. As one moves to the left portion ofthe drawing, the pith dives into the lumber and it is present as a boxedheart condition in the cross section shown in FIG. 7. Note that one ofthe transversely oriented knots seen in FIG. 7 does not break throughthe surface. However, this is shown by grain or fiber angle disturbancesnoted in the upper left portion of FIG. 5 and these are readilydetectable using a scanner of the type described in U.S. Pat. No.4,606,645.

It is relatively easy to determine pith location when wane is presentalong one or more corners of the lumber being examined. Pith is alwaysfound closest to the face which does not contain wane, as is illustratedin FIG. 9.

A convenient way to assign pith position is by considering the piece oflumber being examined as located in the center cell of a grid. The pithlocation can then be assigned to any of the cells. In actual practice,resolution of the type given by FIG. 10 is not normally necessary. Itnormally suffices to say that the pith is closest to the upper face, thelower face or contained with the lumber itself. In the lumber cut in asmall log sawmill pith is located in cells 1, 5 or 9 about 90% of thetime.

FIGS. 11 and 11A illustrate logic diagrams for determining pithorientation. The corners or edges of the lumber are first scanned forwane. If wane is present, the pith can be assigned as being closest tothe face without wane. If no wane is present, additional informationmust be gained and this comes from the scans which measure graindistortion and knot count of the opposed faces. A first check is madefor the presence of spiky faces. If only one spiky face is present, thisis closest to the pith. If two spiky faces are present, this is anindicator of boxed heart. However, if no spiky faces are present, knotcounts on the respective faces are compared. If these knot counts aresignificantly unequal, the face with the largest number of knots isclosest to the pith. If the knot counts are about equal, this is anindicator of boxed heart. Alternatively, if higher resolution is desiredat this point, the additional steps shown in FIG. 11A may be carriedout. If the total knot count on both faces is low, edge knots are thencounted. If the two faces have significantly different edge knot counts,the face with the fewest number of edge knots will be closest to thepith. On the other hand, if the faces have approximately equal numbersof edge knots, the average size of about three of the largest edge knotson each face is compared. Here the side with the largest average edgeknot size is deemed to be the closest to the pith.

Reference to FIG. 1 will show why comparing knot counts on the two facesis an indicator of pith position. The two pieces numbered 1 and 6 haveunequal numbers of knots caused by the presence of small limbs whichbroke off during an early stage of tree growth. Note that the lettersshown within several of the pieces of FIGS. 1 and 2 refer to lumbercross sections having configurations as seen in FIGS. 4A through C.

FIG. 12 diagramatically illustrates a scanner employing the presentmethod. The scanner used is the one described in Matthews et al, U.S.Pat. No. 4,606,645. It employs a low power laser 2 surrounded by a ringof photosensors 4. In the present case eight photosensors are usedequiangularly spaced around the laser. While higher resolution can beobtained by using a greater number of photo sensors, this has not beenfound to be necessary for the present purposes. The laser beam is movedtransversely across the advancing piece of lumber 6 by an oscillatingmirror or lens, not shown, Reflections from the surface of the lumber 6are picked up by the photosensors 4 where they are interpreted intolocalized grain angle relative to three orthogonal axes. Wane sensors 8lie below the viewing plane on each side of the piece of lumber beingexamined. Where there is no wane present, essentially no signal ispicked up by the sensors 8 except for a brief blip at the beginning andend of the scan caused by the eased edges 12 on the lumber. However,when wane is present, the laser beam will be reflected to theappropriate sensor 8 and gives an output as is shown in FIG. 13. Signalsfrom these sensors are preferably fed through logarithmic amplifiers tothat those signals having lower output will be proportionally amplifiedmore than signals of high output. This enables much more preciselocation of the margins of wane area 14 on the lumber. The lumber piece6 also contains a knot 10 and the reflection from this will be clearlypicked up by the ring of scanners 4 as a grain angle disturbance. Forconvenience only one scanner is shown. A similar system will normally beused on the other side of the lumber. Alternatively, a mirror system canbe employed so that alternate scans are made on the top and bottomsurface using a single scanner.

In one installation lumber was passed by a scanner of the type shown inFIG. 12 at a speed of 350 m/min. (1150 ft/min.). Grain patterndistortions indicative of knots and other defects were clearly indicatedby the scanning system. In sawmill tests before the system was fullyoptimized, correct grade was assigned to the lumber 75-80% of the time.At this time the system was not considering such defects as splits,crook and planar skip. A grader could readily observe these defects andoverride the system's assigned grade, if necessary.

When working with a reasonably flat grained or tangentially sawn pieceof lumber there is a very certain way to determine pith locationrelative to the face being viewed. Wood fibers, as a general rule, arelaid down by a growing coniferous tree parallel to its longitudinal axisor pith. However, at the immediate proximity of branches the fibers tendto sweep outward along the branch rather than remain parallel to thepith. This is shown in FIGS. 4C and 12, and especially well in FIG. 5.Note that when a given face of a piece of lumber is being viewed, as inFIG. 12, if the fibers adjacent to a knot converge, or slope in towardthe knot as seen from the viewing location, the pith location is in adirection away from the viewpoint. This is the case for this particularfigure. For the converse situation, if the fibers adjacent to a knotdiverge, or appear to slope in a funnel-shaped configuration, the pithlocation is toward the viewing point. The fiber angle orientation can bereadily determined using a scanner of the type described in U.S. Pat.No. 4,606,645 and the information entered into the logic sequences ofFIG. 11.

Key to the success of the system is the ability to first determine pithposition relative to the faces of the board. With this known, knotorientation is indicated. Then with that knowledge, taken with aknowledge of knot size and position indicated by the scanners, knotcross sectional area and location can be determined with considerableaccuracy.

It will be evident to those skilled in the art that other scanners canbe used for pith position determination besides the scanning systemdescribed in U.S. Pat. No. 4,606,645. For examples, one of the earliernoted gray scale scanners could be used for surface knot determinationalong with a scanner of the type described by Dahlstrom et al for waneindication. However, the inventors believe the system they havedescribed using the scanner of U.S. Pat. No. 4,606,645 to be optimum andto represent the best mode currently known of carrying out theirinvention.

It will be readily apparent to those skilled in the art that manydepartures can be made from the method that has just been describedwithout departing from the spirit of the invention. Thus the inventionis to be considered as limited only by the following claims.

We claim:
 1. A method of determining pith location relative to at leastone face of a piece of lumber which comprises:conveying the lumber pasta scanning station; scanning at least said one face to determine thedirection of wood fiber grain angle relative to three mutuallyorthogonal reference axes adjacent to any knots which may be present,and identifying said pith location from said grain angle information,wherein said pith location is generally away from the scanner positionand within or through the piece of lumber when the fibers adjacent aknot are found to converge toward the knot in a direction toward thescanning position, and pith location is toward the scanning position andoutside the piece of lumber when the fibers are found to converge towardthe knot in a direction away from the scanning position, said pithlocation knowledge being useful for determining knot shape andorientation within the lumber as an aid to grading the lumber.
 2. Themethod of claim 1 where further includes scanning at least said one faceand an opposing essentially parallel second face of the lumber todetermine the presence or absence of wane and its location if present,whereby both the wane and fiber angle information adjacent knots areused to predict pith location.
 3. The method of claim 1 or 2 in whichsaid further scanning step includes computing orientation anddetermining effective diameter and location of any knots detected,estimating cross sectional area of the knots, and then assigning atentative grade to the lumber.
 4. The method of claims 1 or 2 in whichsaid scanning steps include making a multiplicity of longitudinallyspaced apart scans transversely across the faces being scanned.
 5. Themethod of claim 2 in which the scanning steps for wane and wood fiberangle orientation are carried out simultaneously.
 6. The method of claim2 in which the scanning steps are carried out sequentially.